Ever since Lockheedâ€™s unsurpassed SR-71 Blackbird was retired from U.S. Air Force service almost two decades ago, the perennial question has been: Will it ever be succeeded by a new-generation, higher-speed aircraft and, if so, when?

That is, until now. After years of silence on the subject, Lockheed Martinâ€™s Skunk Works has revealed exclusively to AW&ST details of long-running plans for what it describes as an affordable hypersonic intelligence, surveillance and reconnaissance (ISR) and strike platform that could enter development in demonstrator form as soon as 2018. Dubbed the SR-72, the twin-engine aircraft is designed for a Mach 6 cruise, around twice the speed of its forebear, and will have the optional capability to strike targets.

Guided by the U.S. Air Forceâ€™s long-term hypersonic road map, the SR-72 is designed to fill what are perceived by defense planners as growing gaps in coverage of fast-reaction intelligence by the plethora of satellites, subsonic manned and unmanned platforms meant to replace the SR-71. Potentially dangerous and increasingly mobile threats are emerging in areas of denied or contested airspace, in countries with sophisticated air defenses and detailed knowledge of satellite movements.

A vehicle penetrating at high altitude and Mach 6, a speed viewed by Lockheed Martin as the â€œsweet spotâ€ for practical air-breathing hypersonics, is expected to survive where even stealthy, advanced subsonic or supersonic aircraft and unmanned vehicles might not. Moreover, an armed ISR platform would also have the ability to strike targets before they could hide.

Although there has been evidence to suggest that work on various classified successors to the SR-71, or some of its roles, has been attempted, none of the tantalizing signs have materialized into anything substantial. Outside of the black world, this has always been relatively easy to explain. Though few question the compelling military imperative for high speed ISR capability, the astronomical development costs have made the notion a virtual nonstarter.

But now Lockheed Martin believes it has the answer. â€œThe Skunk Works has been working with Aerojet Rocketdyne for the past seven years to develop a method to integrate an off-the-shelf turbine with a scramjet to power the aircraft from standstill to Mach 6 plus,â€ says Brad Leland, portfolio manager for air-breathing hypersonic technologies. â€œOur approach builds on HTV-3X, but this extends a lot beyond that and addresses the one key technical issue that remained on that program: the high-speed turbine engine,â€ he adds, referring to the U.S. Air Force/Defense Advanced Research Projects Agency (Darpa) reusable hypersonic demonstrator canceled in 2008.

The concept of a reusable hypersonic vehicle was an outgrowth of Darpaâ€™s Falcon program, which included development of small launch vehicles, common aero vehicles (CAV) and a hypersonic cruise vehicle (HCV). As structural and aerodynamic technologies for both the CAV and HCV needed testing, Lockheed Martin was funded to develop a series of unpowered hypersonic test vehicles (HTV).

In the midst of these developments, as part of a refocus on space in 2004, NASA canceled almost all hypersonic research, including work on the X-43C combined-cycle propulsion demonstrator. The Darpa HTV effort was therefore extended to include a third HTV, the powered HTV-3X, which was to take off from a runway on turbojet power, accelerate to Mach 6 using a scramjet and return to land.

Despite never progressing to what Leland describes as a planned -HTV-3X follow-on demonstrator that â€œnever was,â€ called the Blackswift, the conceptual design work led to â€œseveral key accomplishments which we didnâ€™t advertise too much,â€ he notes. â€œIt produced an aircraft configuration that could controllably take off, accelerate through subsonic, supersonic, transonic and hypersonic speeds. It was controllable and kept the pointy end forward,â€ adds Leland.

Fundamental lessons were learned, particularly about flight control systems that could maintain stability through the transonic speed regime. Lockheed Martinâ€™s work proved the configuration could â€œtake off without departing,â€ Leland notes. â€œWe were able to drive down the takeoff speed and keep it stable and controllable. We proved all that in a whole series of wind-tunnel tests.â€

Just as importantly, the Skunk Works design team developed a methodology for integrating a working, practical turbine-based combined cycle (TBCC) propulsion system. â€œBefore that, it was all cartoons,â€ Leland says. â€œWe actually developed a way of transforming it from a turbojet to a ramjet and back. We did a lot of tests to prove it out, including the first mode-transition demonstration.â€ The Skunk Works conducted subscale ground tests of the TBCC under the Facet program, which combined a small high-Mach turbojet with a dual-mode ramjet/scramjet, and the two sharing an axisymmetric inlet and nozzle.

Meanwhile, the U.S. Air Force Research Laboratoryâ€™s parallel HiSTED (High-Speed Turbine Engine Demonstration) program essentially failed to produce a small turbojet capable of speeds up to Mach 4 in a TBCC. â€œThe high-speed turbine engine was the one technical issue remaining. Frankly, they just werenâ€™t ready,â€ recalls Leland. This left the Skunk Work designers with a familiar problem: how to bridge the gap between the Mach 2.5 maximum speed of current-production turbine engines and the Mach 3-3.5 takeover speed of the ramjet/scramjet. â€œWe call it the thrust chasm around Mach 3,â€ he adds.

Although further studies were conducted after the demise of the HTV-3X under the follow-on Darpa Mode-Transition program, that fell by the wayside, too, after completion of a TBCC engine model in 2009-10. So, Lockheed Martin and Aerojet Rocketdyne â€œsat down as two companies and asked ourselves, â€˜Can we make it work? What are we still missing?â€™â€ says Leland. â€œA Mach 4 turbine is what gets you there, and weâ€™ve been working with Rocketdyne on this problem for the last seven years.â€

Finally, he says, the two achieved a design breakthrough that will enable the development of a viable hypersonic SR-71 replacement. â€œWe have developed a way to work with an off-the-shelf fighter-class engine like the F100/F110,â€ notes Leland. The work, which includes modifying the ramjet to adapt to a lower takeover speed, is â€œthe key enabler to make this airplane practical, and to making it both near-term and affordable,â€ he explains. â€œEven if the HiSTED engines were successful, and even if Blackswift flew, weâ€™d have had to scale up those tiny turbines, and that would have cost billions.â€

Lockheed will not disclose its chosen method of bridging the thrust chasm. The company funded research and development, and â€œour approach is proprietary,â€ says Leland, adding that he cannot go into details. Several concepts are known, however, to be ripe for larger-scale testing, including various pre-cooler methods that mass-inject cooler flow into the compressor to boost performance. Other concepts that augment the engine power include the â€œhyperburner,â€ an augmentor that starts as an afterburner and transitions to a ramjet as Mach number increases. Aerojet, which acquired Rocketdyne earlier this year, has also floated the option of a rocket-augmented ejector ramjet as another means of providing seamless propulsion to Mach 6.

Although details of the proposed thrust-augmentation concept remain under wraps, Leland says a large part of a successfully integrated mode-transition design is the inlet. â€œThatâ€™s because you have to keep two compressor systems [ramjet and turbine] working stably. Both will run in parallel,â€ he adds.

Lockheed has run scaled tests on components. â€œThe next step would be to put it through a series of tests or critical demonstrations,â€ Leland says. â€œWe are ready for those critical demonstrations, and we could be ready to do such a demonstration aircraft in 2018. That would be the beginning of building and running complete critical demonstrations. As of now, there are no technologies to be invented. We are ready to proceedâ€”the only thing holding us back is the perception that [hypersonics] is always expensive, large and exotic.â€

The 2018 time line is determined by the potential schedule for the high-speed strike weapon (HSSW), a U.S. hypersonic missile program taking shape under the Air Force and Darpa (see page 36). â€œWe can do critical demonstrations between now and then, but we donâ€™t believe it will be until HSSW flies and puts to bed any questions about this technology, and whether we can we truly make these, that the confidence will be there.â€ In spite of the recent success of demonstration efforts, such as the X-51A Waverider, Leland observes that â€œhypersonics still has a bit of a giggle factor.â€

The timing also dovetails with the Air Force hypersonic road map, which calls for efforts to support development of a hypersonic strike weapon by 2020 and a penetrating, regional ISR aircraft by 2030 (AW&ST Nov. 26, 2012, p. 40). Key requirements for the high-speed ISR/strike aircraft is the ability to survive a â€œday without spaceâ€â€”communication and navigation satellitesâ€”and to be able to penetrate denied areas. With a TBCC propulsion system, the Air Force has pushed for increasingly greater speeds since defining Mach 4 at initial planning meetings in December 2010. The latest requirements are thought to be at least a Mach 5-plus cruise speed and operation from a conventional runway.

The path to the SR-72 would begin with an optionally piloted flight research vehicle (FRV), measuring around 60 ft. long and powered by a single, but full-scale, propulsion flowpath. â€œThe demonstrator is about the size of the F-22, single-engined and could fly for several minutes at Mach 6,â€ says Leland. The outline plan for the operational vehicle, the SR-72, is a twin-engine unmanned aircraft over 100 ft. long (see artistâ€™s concept on page 20). â€œIt will be about the size of the SR-71 and have the same range, but have twice the speed,â€ he adds. The FRV would start in 2018 and fly in 2023. â€œWe would be ready to launch the SR-72 shortly after and could be in service by 2030,â€ Leland says.

According to Al Romig, Skunk Works engineering and advanced systems vice president, â€œspeed is the new stealth.â€ This is perhaps just as well, given the inherent challenges involved in reducing the signature of hypersonic vehicles. With large engine inlets and aerodynamic requirements overriding most considerations, the SR-72 concept shows little in the way of stealthy planform alignment. Although the surfaces could be coated with radar-absorbing material, the requirement for thermal protection along sharp leading edges is likely to be a complicating factor. Like the HTV-3X, the vehicle may also feature hot metallic leading edges and a â€œhot/warmâ€ metallic primary structure designed to handle the high thermal flux loads.

The deep nacelles, mounted close inboard, indicate the â€œover-under combined cycleâ€ engine configuration outlined for the HTV-3X, as well as integrated inward-turning turbo-ramjet inlets. â€œOne of the differences with this demonstrator compared to the HTVâ€‘3X is that with that, we were limited to small turbines with a low-drag design,â€ Leland says. â€œWith fighter engines, we accelerate much more briskly. Itâ€™s a significant improvement in adding margins. It is also very important [that] you have a common inlet and nozzle because of the significant amount of spillage drag in the inlet and the base drag in the nozzle.â€

Aerodynamically, the forebody appears to be shaped for inlet compression at high speed, but without the characteristic stepped â€œwave-riderâ€ configuration of the X-51A. â€œWe are not advocates of wave riders,â€ Leland says. â€œWe found that, in order for a wave rider to pay off, you have to be at cruise and be burning most of your fuel at cruise. But these designs burn most fuel as they accelerate, so you want an efficient vehicle that gets you to cruise. You end up with a vehicle that is hard to take off and land, has little fuel volume and high transonic drag.â€

The planform is characterized by chines that blend into a sharply swept delta extending back roughly halfway along the hump-backed fuselage. The chine and delta are likely designed to provide increased directional stability as well as a larger amount of lift at high cruise speeds. Outboard of the engine inlets, the leading-edge angle abruptly aligns with the fuselage before the wing extends into a trapezoid. The angle of the cranked wing would provide vortex lift to assist with low-speed flight.

The SR-72 is being designed with strike capability in mind. â€œWe would envision a role with over-flight ISR, as well as missiles,â€ Leland says. Being launched from a Mach 6 platform, the weapons would not require a booster, significantly reducing weight. The higher speed of the SR-72 would also give it the ability to detect and strike more agile targets. â€œEven with the -SR-71, at Mach 3, there was still time to notify that the plane was coming, but at Mach 6, there is no reaction time to hide a mobile target. It is unavoidable ISR,â€ he adds. Lockheed envisages that once the FRV has completed its baseline demonstrator role, it could become a testbed for developing high-speed ISR technologies and supporting tests of the SR-72â€™s weapons set, avionics and downlink systems.

â€œIt is time to acknowledge the existence of the SR-72 because of the HSSW going forward,â€ says Leland. Together with the strategic â€œpivot to the Pacific,â€ the concept of high-speed ISR is â€œstarting to gain traction,â€ he notes. â€œAccording to the hypersonic road map, the path to the aircraft is through the missile, so now it is time to get the critical demonstration going.â€ These would test individual elements of the propulsion system, which would then be integrated for the full-scale FRV evaluation.

â€œWe have been continuing to invest company funds, and we are kind of at a point where the next steps would require large-scale testing, which would significantly increase the level of investment weâ€™ve had to make to-date. Between Darpa and the Air Force, it would be highly likely theyâ€™d have to fund the next steps,â€ Leland says. The FRV will also give the Skunk Works a better idea on overall development costs, he adds.

As for rumors of an existing high-speed ISR aircraft, Leland is dismissive. â€œItâ€™s been almost 20 years since the SR-71 was retired. If there was a replacement, theyâ€™ve been hiding it pretty well,â€ he says.

It would be great to know the actual service ceiling of the aircraft. But it will definitely be faster than any planned Russian fighter aircraft. So they will have to start making plans for a mach 6+ fighter jet like they did with Mig-25 and Mig-31 and get it in service in 3 or 4 years after the SR-72 is operational.

This design looks FAR less technical than the SR-71. The Black bird also had Ram jets that by-passed the jet engine, the SR-71 wasted far less space by incorporating the Ram and Turbine solution in one housing which consumed much less space..The Engine to body ratio of SR-71 shows a much larger engine to body weight. While the SR-72 looks like mere wet dream with disproportionate sizes and failure of imagination..

Truth is what ever you can think of they already thought of in the 60s and 70s, You cant do much interms of design development, only newer materials can be used to reduce weight, they played with FAR FAR more designs and brains storms during the cold war. So you young boys cant afford that.

The higher you go, the less capable your speed is in the operational envelope. SAMs will catch you since radars will find you sooner. And today's Russian SAMs are theoretically capable of stopping Mach 15 objects which they are bringing into practical use, let alone mach 6 objects.

Of course, the SR-72 will most probably be more stealthy than a F-22, but that's a different topic considering radars in 2030 would also be different in capability.

The higher you go, the less capable your speed is in the operational envelope. SAMs will catch you since radars will find you sooner. And today's Russian SAMs are theoretically capable of stopping Mach 15 objects which they are bringing into practical use, let alone mach 6 objects.

Of course, the SR-72 will most probably be more stealthy than a F-22, but that's a different topic considering radars in 2030 would also be different in capability.

Today's cruise missiles and quasi-ballistic missiles can already do that with conventional technology. And there are missile systems being designed to stop such missiles today. Brahmos 2 will perform the same but with a SCRAMJET, and it will be ready for operation within the decade.

Google SSTO and see where the world is really headed. 2030 should give a far clearer picture for the future of this technology, including the existence of real aircraft with near unlimited range.

Mach 6 looks like a big number today, but that's not enough if you want a survivable aircraft in enemy territory.

SR-71 used as recently as 1998, over Bosnia, before retired for unmanned drones, which is cheaper and safer......they still need a plane over local area of interest, satellite has limitations, like cannot loiter over area .........

The SR-72 will travel at six times the speed of sound—the fastest military jet ever made—and fly as high as 80,000 feet.

Born in the spy-vs.-spy cauldron of the Cold War, the iconic SR-71 “Blackbird” remains the fastest air-breathing military aircraft the world has known. It flew so high and so fast that enemy defenses were powerless to intercept it. Eventually, satellite technology and advanced radar eroded its advantage. In 1998, the U.S. Air Force retired it. Now, with regional threats growing and portable surface-to-air missiles evolving, engineers have once again set out to build the fastest military jet on the planet.

This time, it will take the form of a 4,000-mile-per-hour reconnaissance drone with strike capability. Known as the SR-72, the aircraft will evade assault, take spy photos, and attack targets at speeds of up to Mach 6. That’s twice as fast as its predecessor.

Aeronautical engineers at Lockheed Martin and Aerojet Rocketdyne have been designing the SR-72 at their Skunk Works black site in California for the past several years. It will require a hybrid propulsion system: a conventional, off-the-shelf turbo jet that can take the plane from runway to Mach 3, and a hypersonic ramjet/scramjet that will push it the rest of the way. Its body will have to withstand the extreme heat of hypersonic flight, when air friction alone could melt steel. Its bombs will have to hit targets from possibly 80,000 feet. Lockheed says the craft could be deployed by 2030. Once it is, the plane’s ability to cover one mile per second means it could reach any location on any continent in an hour—not that you’ll see it coming.

HOW RAMJETS WORK
Ramjets forgo the big rotary compressors needed on turbojets and instead rely on their own forward motion to compress air. First, air is scooped into an inlet and compressed as it funnels into a diffuser. The diffuser also slows the air to subsonic speeds for easier combustion. From there, air and fuel are fed into a combustion chamber and ignited. Finally, an exhaust nozzle accelerates the resulting burst of hot, expanding air, producing massive thrust.

PROPULSION
Turbojet engines can take a plane from runway launch to about Mach 3; speeds faster than that require an air-breathing ramjet, which compresses high-speed air for combustion, but which typically begins operating at about Mach 4. To bridge the gap, engineers are developing a hybrid engine that can operate in three modes. The aircraft will accelerate to about Mach 3 under turbojet power, switch to ramjet power to take it to about Mach 5, and then switch again to scramjet mode, which uses supersonic air for combustion.

SKIN
Aerodynamic friction at speeds exceeding Mach 5 will heat an aircraft’s exterior to 2,000 degrees. At that point, conventional steel airframes will melt. So engineers are looking at composites—the same kinds of high-performance carbon, ceramic, and metal mixes used for the noses of intercontinental ballistic missiles and space shuttles. Every joint and seam must be sealed: Any air leak at hypersonic speed, and the in-rushing heat would cause the aircraft to collapse. (That’s what doomed the space shuttle Columbia).

AIRFRAME
The stresses on a plane shift as it travels through subsonic, supersonic, and hypersonic speeds. For instance, when a jet is accelerating through subsonic flight, the center of lift moves toward the back of the aircraft. But once the craft hits hypersonic speeds, drag on the plane’s leading egdes cause the center of lift to move forward again. If the ceter of lift gets too close to the center of gravity it can cause dangerous instability. The plane’s shape must tolerate these changes, and more, to keep the craft from tearing apart.

PAYLOAD
Lockheed describes the SR-72 as an intelligence, surveillance, reconnaissance, and strike platform, but its exact payload is secret. Most likely, it hasn’t yet been invented. Taking spy photos or dropping bombs at Mach 6 will require extraordinary engineering. It will require hundreds of miles to make a turn. It will need powerful guidance computers to line up targets, 80,000 feet below. Also, you can’t just open a bomb bay at 4,000 miles per hour. The SR-72 will need new sensors and weapons to operate at such high speeds.